About one-tenth of the mass of our Galaxy is interstellar gas, about half of it atomic hydrogen and helium and most of the rest molecules-mainly molecular hydrogen but over a hundred other compounds as well. Because molecular hydrogen has no strong transitions at radio or millimeter wavelengths, the molecular component was largely unsuspected until about 35 years ago when the trace molecule carbon monoxide (CO) was first detected in space. Since then, large-scale CO surveys of the Galaxy have shown that the molecular gas is mainly concentrated in large, cold, relatively dense clouds, some containing up to a million times the mass of the Sun. All star formation takes place within the denser cores of these clouds, where the process is largely obscured optically by trace amounts of dust. Astronomers in the R&G Division play a leading role in star formation research because the radio and infrared telescopes they employ can penetrate the dust to reveal both the initial conditions and early stages of star formation.

The white lines outline the constellation of Orion the Hunter. The slightly pinkish blob in the Hunter's sword is the Orion Nebula, the nearest and most well-studied region of massive star formation. Flashing on and off is a map of the distribution of dense molecular gas in Orion, obtained with the CfA millimeter-wave telescope. Note how the Orion Nebula lies in the densest part of the Orion molecular cloud.